The European Union (EU) has agreed a conditional deal on how to help other nations fight global warming, ahead of a key climate summit.
The European Union agreed climate change would need at least 100bn euros, around £90bn a year by 2020.
Gordon Brown said the deal, which came after a deadlock on cost sharing was broken, was a “bold proposal”.
However, some green groups criticised the deal, saying it was not nearly enough.
The accord had been threatened by a coalition of nine poorer EU nations, which argued that richer countries should pay more.
To meet the concerns, the initial funding will be voluntary and no cost targets for individual EU nations were announced.
Details of how the burden will be shared will be sorted out later by a working group.
Earlier EU leaders agreed a deal designed to secure the ratification of the Lisbon Treaty, which aims to streamline decision-making and bolster the bloc’s role on the world stage.
Vaclav Klaus, president of the Czech Republic, the only country that had been holding out on ratification, said it would no longer raise objections.
Mr Brown, announcing the climate deal, said the EU discussions had been a success.
“We were aware that if we did not come together to make progress, the possibility of a deal would be a lot less likely.”
He said: “Europe is leading the way with these bold proposals – do not allow years to go by without action.”
The EU said the amount to come from public funding from all countries to meet the estimated 100bn euros a year needed by 2020 would be between 22bn and 50bn euros a year.
However, it did not fix the EU’s contribution, saying it would only pay its “fair share”.
But Mr Brown did announce a “fast track” scheme to reduce carbon emissions, with the richest countries providing development finance to the poorest.
This would come in soon after the Copenhagen summit and would cost 5bn to 7bn euros immediately, to come from all richer countries.
He insisted that all these funding targets would be conditional on other richer countries making funding offers and on developing countries showing how they would spend the money.